The technique of down-the-hole (DTH) percussive hammer drilling involves the supply of a pressurised fluid via a drill string to a drill bit located at the bottom of a bore hole. The fluid acts to both drive the hammer drilling action and to flush rearwardly dust and fines resultant from the cutting action, rearwardly through the bore hole so as to optimise forward cutting.
Typically, the drill assembly comprises a casing extending between a top sub and a drill bit that, in turn, is releasably coupled to a drive component (commonly referred to as a chuck or drive sub). Drilling is achieved via a combination of rotation and axial translation of the drill bit. Rotation is imparted to the drill bit from the drive sub via intermediate engaging splines. The axial percussive action of the bit is achieved via a piston that is capable of shuttling axially between the top sub and the drill bit and is driven by the pressurised fluid to strike a rearward anvil end of the bit. In some embodiments, a foot valve extends axially rearward from the drill bit to mate with the piston during its forwardmost stroke to control both the return stroke and provide exhaust of the pressurised fluid from the drill head that acts to flush rearwardly the material cut from the bore face. Example DTH hammer drills are described in WO 2008/051132, WO 2013/104470 and U.S. Pat. No. 6,131,672.
Conventionally, the drill bit is retained at the assembly and in contact with the drive sub via a retainer ring. The retainer ring extends around a rearward end of the drill bit shaft and is configured to abut a radially projecting shoulder positioned at the axial rearward end of the bit shaft. Such a configuration prevents the drill bit from falling out of the hammer assembly during flushing or when the hammer assembly (and the drill bit) are loaded into or extracted from the borehole. Example retainer ring assemblies are described in U.S. Pat. No. 5,803,192; US 2007/0089908; EP 1462604 and WO 2001/21930.
However, these conventional retaining assemblies are disadvantageous for a number of reasons. In particular, in an attempt to minimise wear at the drive sub splines, a portion of the pressurised fluid delivered to the drill bit is diverted to the radially outer region of the bit and into contact with drive sub splines. Typically this diverted fluid flow passes between the radially inner surface of the retainer ring and the outer surface of the drill bit shaft. Accordingly, conventional retainer rings include flow path channels or otherwise have a structure to allow the lubricating fluid to reach the axially forward drive sub splines. However, these airflow passageways at the radially inner region of the retainer ring reduce the area contact of the ring with the abutment shoulder of the drill bit. Accordingly, conventional bit retainers are weakened and drill bit retention is compromised. Accordingly, what is required is a drill bit retaining assembly that addresses the above problems and in particular allows delivery of a desired volume of lubricating fluid to the splines whilst providing a secure and reliable retention of the drill bit.